The present invention relates to electronic ignition systems for internal combustion engines, and particularly to a device for the automatic variation of the spark advance in such an electronic ignition-system.
A number of different arrangements for varying the spark advance in electronic ignition systems for internal combustion engines are already known; this invention relates particularly to electronic systems for the automatic variation of the spark advance, which systems generally comprise an ignition spark discharge control circuit, and a circuit for varying the instant of discharge, which circuit is controlled by reference means located on a rotating element, generally the flywheel of the internal combustion engine.
Such reference means usually comprise a series of notches and corresponding teeth, of ferromagnetic material, spaced around the circumference of the flywheel or other rotating element in such a manner as to provide a detectable varying magnetic flux as the element rotates; a system for detecting the variation of the flux comprising, for example, one or more electromagnetic pick-ups, which cooperates with the ferromagnetic reference means on the rotating element so as to detect the speed of rotation and phase of the rotating element, and finally, electronic circuits responsive to the output signal from the pick-up for controlling the discharge of the ignition spark, operating to increase the spark advance as the speed of the engine increases.
Known ignition timing control systems are generally constructed in such a manner as to provide the spark at a minimum angular advance setting, in relation to the point of maximum compression of the charge in the cylinder, corresponding to a selected minimum speed of the engine, and the ignition advance increases linearly with the speed of the engine to reach a constant fixed value when the engine reaches a predetermined maximum speed.
The form of the ignition advance curve achieved by such known devices, however, only approximates to the actual curve required for the optimum functioning of the engine, which curve is usually of more complex form, and is different for different types of engine.
In order to approximate more closely the curves required in practice for certain types of engines, it is possible to use circuits which produce curves in which the part which varies linearly with the speed of the engine, is substituted by two straight lines of differing slope.